In an enclosed ecosystem such as an aquarium, disease does not just occur spontaneously from no-where. It is always induced either by an external source such as live foods or a result of an environmental factor such as poor tank conditions.

Visible diseases such as parasites or worms are straightforward to diagnose and one should always seek to identify the source of the infection to prevent a reoccurrence. Unfortunately, in the majority of fish deaths, the victim dies displaying no signs of obvious infection. Most of these deaths can be attributed to non-living detrimental environmental factors.

Environmental factors include providing the basic requirements for certain species to survive such as the correct temperature, hardness and pH but also involves sustaining these factors through good tank maintenance and water quality. Managing ammonia, nitrates, pH and good filter care are some of the keys to a healthy aquarium.

Some of the more common factors that adversely affect fish are described below.

Nitrogen

This encompasses all products of the nitrogen cycle - ammonia (and ammonium), nitrite and nitrate. Ammonia is toxic at levels as low as 0.2 mg/l (parts per million), nitrite will affect fish from levels above 1 mg/l and less toxic nitrate has adverse affects in concentrations of 100mg/l upwards depending on the species of fish.

Managing the basic procedures of fish keeping will control these elements. Undertake good aquarium maintenance coupled with regular water changes and a controlled feeding regime.

Signs of Nitrogen problems include increased respiration, cloudy water, rotting debris on the substrate and a yellow tint to the water indicating and ammonia build up. The solution is to reduce feeding and perform regular water changes until the situation is controlled.

Chlorine

Tap water is often treated with chlorine to kill microorganisms. Chlorine is a disinfectant that disappears if left to stand or is aerated for 24 hours. It can also be removed by a commercial product such as Tetra Aqua Safe. These products generally contain sodium thiosulphate but many also have added protective colloids to reduce the stress on the fish caused by raw tap water.

Chlorine reacts with any ammonia in the water to form chloramines. More water companies are now using chloramine as a disinfectant because they take a longer time to disperse than chlorine alone but this unsuitable for the fish keeper. Again commercial products are available to remove chloramines or they can be dispersed by aerating for up to 5 days or by using carbon filtration.

Chlorine attacks the gills and can lead to death through oxygen starvation. Chlorine can prove fatal at levels as low as 0.1 mg/l. It is therefore imperative to treat tap water that contains chlorine and chloramine before introducing it to the aquarium.

Oxygen

A lack of oxygen, vital for respiration, can occur from decaying foods, rotting debris and over stocking - all factors that consume oxygen. Additionally, higher temperatures can lead to oxygen deficiencies as less oxygen can be dissolved in warmer waters.

Initial symptoms include increased respiration, fish gasping at the surface and signs of stress. Prolonged oxygen starvation will lead to outbreaks of disease and death.

The solution is to aerate the water and remove decomposing debris along with a water change. ensure that stocking levels in the aquarium are managed and balance the aquarium with plants which will produce oxygen at night.

Excess oxygen can produce gas bubble disease. Symptoms of gas bubble disease include clear blisters forming beneath the skin most predominantly around the head and on the eyes.

Excessive oxygen can occur from an over saturation of oxygen in the water created by heavy plant and algae growth. The saturation of oxygen in the water transfers through the gills into the fishes blood and it is this that causes the visible blisters. In extreme cases this can cause death. The simplest cure is to reduce sunlight and thus slow plant growth although the gas bubbles, unable to escape, may remain on the fish

Acidosis and Akalosis

This is the effect of fluctuating pH values. Affected fish display excess mucus, inflammation and erosion of the gills and sometimes bleeding. Frequent darting and unnatural jumping in an attempt to leave the aquarium is also linked with this problem.

An immediate water change is the best solution taking care to adjust the pH level of the new water to suit the requirements of the fish. Frequent monitoring of pH will prevent a reoccurrence.

Metals

Iron, copper and lead all breakdown the protective slimy coating that is designed to protect fish. They can also cause gill damage resulting respiratory problems and death.

Iron is required by many tropical plants and is sometimes naturally present in the aquarium. However, this iron has to be present with other organic substances and not simply an iron solution in the water. Iron concentration below 1.5mg/l are usually considered safe.

Copper may build up in water which has been standing in copper pipes or hot water cisterns, particularly in soft water areas. Fish are sensitive to copper in excess of 0.5mg /l and can be come stressed by levels of 0.1 mg/l. As a precaution, I recommend running some waste before using tap water for you aquarium and I advise against using any hot tap water as it may have been held in a hot water cistern (usually made of copper) for some time. Additionally, hot water cisterns collect a lot of debris particularly carbonates which are undesirable to soft water fish.

Detergents

Household cleaners and soap have a similar effect to metals described above. Always rinse your hands before working in the aquarium and take care when polishing the front glass or surrounding furniture to ensure that household cleaners do not enter the tank.

Hydrogen Sulphide

A poisonous gas that smells like rotten eggs. Hydrogen sulphide binds the iron in the bloodstream blocking the absorption of oxygen. Aside from the smell, symptoms include a reddening of the gills and, rapid breathing or gasping at the surface.

The cause, usually excess feeding and poor cleaning of the substrate should be rectified immediately and a 30 percent water change performed.